Rethinking the natural water cycle

Groundbreaking water research out of the University of Saskatchewan has just been published in the international science journal Nature.

Researchers Jaivime Evaristo and Dr. Jeffrey McDonnell of from the U. of S. and Scott Jasechko of the University of Calgary have taken a new look at the hydrological cycle, something that’s been pretty well established in science for some time.

“We all learn in Grade 10 about the hydrological cycle,” McDonnell said in a telephone interview. “The idea that water evaporates from the earth’s surface, it forms clouds, the water stays in the atmosphere on average a week or so globally, and then that comes back to earth as rain or snowfall, and water then goes into the ground, or runs off into streams, and the cycle keeps going around and around.”

How swiftly and intensely that cycle circulates depends on the location in the world. “If you’re in the tropics, it’s usually a very fast, intensive cycle, and then in a place like Saskatchewan, it’s a less intense cycle. Water gets locked up in snow and frozen soil, and it’s released slowly. But everywhere on earth that cycle, we thought, is one that’s ‘what goes up comes down, and what comes down ultimately leaves the system in the same way.’”

In that framework “when the precipitation went into the ground, we thought of it going into one big tank, reservoir or bucket.” That soil water would be water for agriculture and plants, and then what didn’t plants use would go into groundwater —streams, lakes, and the oceans.

A new view

The new research, though, suggests something a little bit different happens when the water is in the soil, underground. It looks as though there is a compartmentalization of part of the hydrological cycle, where water is being split up into a couple of different pools, says McDonnell. “What we found is that some water that goes into the ground goes into one pool — that bucket of water — and that water is almost stuck in the system.

“That’s the water the plants seem to be interested in using. The water that’s more mobile, and goes down and recharges groundwater gets into the stream flow, that water doesn’t seem to be connected to the water that our plants are extracting.”

The findings are the first on a global scale that support the “two water world hypothesis” — the idea that there is “a plant water world and a more mobile water world for groundwater and stream flow.”

This all seems a bit mind-boggling considering that all water is H2O. But as McDonnell says, “it’s all H2O, but I can say something about what type of H and what type of O using isotope fingerprinting techniques (the chemical fingerprint of the water).” This work also seems tied to the characteristics of the soil underground, considering that the two water worlds are both under the earth in similar ways. It’s not that the water plants use is closer to the earth’s surface and that groundwater is further underground, rather the different chemical compositions of the water interact with the soil — which has particular qualities too. Some soil is “really tightly bound” and that soil seems to be carrying the water that plants use, while soil that has “bigger pores — which might be, at the extreme, decayed root channels or an animal burrow” that’s the water that feeds into streams and the like. That water doesn’t seem interesting to plants. All of this adds up to a major rethinking of how water cycles.

Research in practice

This new study could affect farmers’ watering practices down the road. “One thing that we’ve found is that crops also seem to partition their water, like we’re seeing in forests and the like,” says McDonnell.

McDonnell says Chinese researchers who have looked at the “two water world” idea in corn fields had similar findings: the corn fields were “using water tightly bound in the soil, while the water that was in the streams and in groundwater looked very different from the water the corn was using.”

Because agricultural crops seem to be following the same new rules, further research could help rethink irrigation practices on farmers’ fields. “How we irrigate, the intensity, the magnitude, and how we think about irrigation water — which is kind of like rainfall” are all questions that could come up later on in research, ultimately allowing farmers to maximize irrigation.

Earth’s system is complicated. McDonnell says, “there are just so many things we don’t understand. This is a very small example of something we thought we understood, but with new techniques we’ve realized that we need to write a new chapter on the hydrological cycle.”

With droughts making the news all summer long, and water crises around the world, this new finding could really have positive implications on “the precious resource we call water,” says McDonnell. “Knowledge is power and it’s pretty tough to manage a resource if you don’t know how it works.”